Broadcasting has been successful throughout the world in bringing both entertainment and information to mass audiences. The history of broadcasting began almost a century ago with radio, while the history of TV stretches back to the 1930's.
The latest step in broadcasting is the digitalisation of both radio and TV. Digital radio has not yet gained widespread acceptance on the market. However, many hope that digital TV will bring new benefits and services to the consumer and, as a result, generate new revenue streams for the broadcasting industry. The basic concept of the TV service itself has, however, not changed much. Rather, the TV services continue as before, even where they have become digital.
The later half of 1990's saw a boom in use of the Internet. A whole set of new services and content became available to the consumers during a short, revolutionary and hype intensive period. E-commerce, Internet Service Providers (ISPs), portals, eyeballs game, dotcom companies and even the new economy were introduced during that period. Developments in both access technologies (e.g. ADSL) and coding technologies (e.g. MPEG-2 streaming) have made it possible to bring rich media content, such as video content, into homes via the Internet. In spite of these technology and market breakthroughs, media houses have been reluctant to distribute their content via the Internet due to its “free-of-charge” nature and the direct threat of piracy. Furthermore, despite its great popularity, the Internet has yet to challenge the role of traditional media as the primary advertisement platform.
Impulsive interference is observed to cause difficulties in broadcast reception. This interference may be produced by ignition sparks from vehicles or various household appliances like hair-dryers, vacuum cleaners, drilling machines etc. The cheapest models of these appliances often have insufficient interference suppression. Also, for the same reason, a single pulse, or even a burst of pulses, is produced when switching on or off any device connected to the power line. These could be electrical heating devices, thyristor dimmers, fluorescent lamps, refrigerators etc. The effect of impulsive interference has to be taken into consideration, especially for indoor reception with a simple omnidirectional aerial. The field strength of a broadcast signal, especially for a portable device situated indoors, can be quite low and further weakened by multipath reception. For fixed reception, insufficient cable shielding in inhouse signal distribution means often reduces the benefit of a roof aerial, making the signal reception sensitive to impulsive interference.
A prior approach for reducing the effects of impulsive noise is based on clipping the impulse bursts, in which corrupted signal samples are given the value corresponding to the amplitude of a clipping level, while retaining their phase. Alternatively, the clipped values may be set to a value of zero because the affected samples are known to be unreliable in any case. An example of an approach along these lines is described in patent application EP 1 043 874 A2. However, this approach leaves samples that have been corrupted but not clipped untouched, so that noise-affected samples having amplitudes below the clipping level remain. This leads to a poor signal-to-interference ratio, especially if the burst power is high. Approaches based on clipping do not detect, and therefore cannot remove, impulse noise with signal levels below the clipping level, limiting the capability of these methods.
Another approach for eliminating impulsive noise is to blank all the samples that are known to be corrupted, i.e. those samples belonging to an interference burst period. The knowledge of impulse position and duration may be derived, for example, by monitoring an incoming signal for samples with amplitudes exceeding a predetermined threshold or clipping level. One such approach is presented in Sliskovic, M: Signal processing algorithm for OFDM channel with impulse noise, The 7th IEEE International Conference on Electronics, Circuits and Systems (ICECS 2000), Volume: 1, 2000, Page(s): 222-225 vol. 1, where, in order to improve performance, estimates of the values of the original signal for the blanked samples are derived using information from guard band samples. Unfortunately, this requires the solution of general complex system equations, which is cumbersome. In addition, relying on the spectrum part in the guard band is undesirable in systems where many carriers are received through a noisy channel, as the original values of missing samples cannot be determined reliably. Furthermore, this method does not overcome the problem of identifying corrupted samples having amplitudes below the threshold or clipping level.
Thus, there is a need for a receiver that can withstand high levels of interference and provide high data reception quality that is suitable for receiving a data signal comprising a large number of carriers.